VI.9 Regulating Services: A Focus on Disease Regulation Peter Daszak and A. Marm Kilpatrick OUTLINE 1. Infectious diseases as disrupters of ecosystem services 2. Diseases as providers of regulating ecosystem services 3. Ecosystem regulation of infectious diseases 4. Valuing the economic impact of pathogens and their ecosystem services Over the past few decades there has been an explosion of interest in the ecology of infectious diseases and their roles in ecosystem function. Many studies have focused on the dynamics of pathogens within human, other animal, and plant populations and their role in causing mass mortality events and population declines. Other researchers have focused on diseases that are increasing in incidence or geographic or host range—‘‘emerging’’ infectious diseases—of humans, wildlife, and plants. However, relatively few researchers have approached disease ecology in the context of ecosystem services, where diseases, parasites, or pathogens perform functions potentially useful to humans. In this chapter, we review the literature on three aspects of parasites and pathogens in the ﬁeld of ecosystem services. The ﬁrst is probably the most well known: their role in morbidity and mortality to their hosts, through which they disrupt the host’s ability to provide an ecosystem service, i.e., disrupting the survival or life-history success of ecosystem service providers. The second is the role of pathogens as regulating service providers by suppressing populations of pest species, resisting invasion, and acting as biocontrol agents. The third aspect is poorly understood but a subject of growing interest: rather than parasites performing the service per se, it is the role of species, communities, and biodiversity in regulating the risk of infectious diseases to people, i.e., performing a regulating service, that reduces disease risk. The thrust of our chapter is to review the state of the ﬁeld, and we have paid particular attention to highlighting those areas where future research is likely to be most fruitful and to identifying strategies to take the ﬁeld forward. We have therefore added a fourth section that discusses efforts and strategies to estimate the value of pathogens and the cost of their impact on natural capital and ecosystem services. GLOSSARY density dependent. A density-dependent process varies with the population density of the species concerned . For instance, below a certain host population size, parasitic infections may not occur (there are not enough hosts for the parasite to be transmitted between them), whereas above a certain host population size, parasitic infections may become prevalent. The probability of any individual host getting infected depends on the density of surrounding hosts. emerging infectious disease. A disease that has recently and signiﬁcantly increased in impact, in the number of cases it causes, or in its geographic range; a disease that is caused by a newly evolved pathogen or has recently been transmitted from one species to another to result in an outbreak in the new host species. parasite. An organism that resides within or on, and is nutritionally dependent on, another organism. In this article, we include all forms of infectious microbes , including viruses, prokaryotes (e.g., bacteria ), and eukaryotic parasites (e.g., roundworms). pathogen. An infectious agent or parasite that causes illness in its host, usually deﬁned as clinical illness, i.e., causing signiﬁcant pathology or damaging physiological change. 1. INFECTIOUS DISEASES AS DISRUPTERS OF ECOSYSTEM SERVICES Infectious diseases have been reported to be the cause of morbidity and mortality in a range of key ecosystem service providers (ESPs) (table 1). In these cases, pathogens act as ‘‘mediators’’ of the loss of ecosystem services and effectively perform an ‘‘ecosystem disservice .’’ The impact of pathogens is greatest when they cause population declines of keystone species or ecosystem engineers. For example, the death of one-third of the Serengeti lion population caused by canine distemper (a disease introduced via domestic dogs) had a disproportionate impact on the ecosystem. Pathogens sometimes spread rapidly through highly susceptible host populations, which include host populations that have not evolved in the presence of the pathogen. Introduced pathogens may also impact abundant species at lower trophic levels and have similarly dramatic effects on ecosystem services. Human attempts to control or manage diseases can also have unanticipated ancillary impacts (either positive or negative) on ecosystem services. The introduction of rinderpest into Somalia in 1889 with imported domestic cattle led to a pan-African outbreak and widespread loss of livelihood as it caused the death of millions of domestic and wild ungulates and ecosystem collapse over large areas. But it also provides an...

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